I've personally never done one, but I will say this. "an empty table with my business cards and some flyers" is the worse thing you can do. How many tables have you passed up because they just look like a marketing/sales man gimmick? Me, I've passed up hundreds!
You need to show a presence, you need to have something that captures and holds someone's attention for at least 5 seconds. Get past the first 5 seconds and you will have a potential client/customer/referral.
Posters of sites is a REALLY REALLY good idea. Line them up, make sure they are appealing to the eye, colorful, etc. Don't place flyers or business cards on the table. Fill it with photos of screenshots if the sites you did. Have them on the table for people to browse through, all while they are there you can ask them questions, get involved, anything that keeps them there for at least 5 seconds.
When they are getting done, ask if they would like a business card or flyer.
Another idea, have a raffle for 25% off for one lucky person/customer. However! Do not put this up front and center. Get the person to come to you not just for the raffle, but because they show interest. While they are there, ask if they would like to enter the raffle and give them a ticket and pen.
Just my 2 cents